The invention relates to a device for smoothing and cooling or for cooling of a sheet material, especially a film. The sheet may be processed into plates made from thermoplastics. The sheet of film is extruded by a sheet die extruder.
DE 35 30 309 A1 discloses a procedure and a device for continuously manufacturing thermoplastic sheets. A known dual-band press is proposed as a device to carry out the procedure of calibrating and smoothing. In the reaction zone, the surface pressure is applied to the thermoplastic sheet. Here, a device is present to at least partly take up heat from the thermoplastic sheet. The press bands are arranged one above the other, and are respectively guided via two deflecting rolls. The plastic melt exits the extruder die on a press belt or is introduced into the gap between the deflecting rolls. A pressing plate is arranged between the two deflecting rolls. Furthermore a mechanical pressure transmission is provided. Here, stationary metal rolls are mounted between the pressing plate and the lower surface of the press belt. The pressing plate is adjusted, with the help of hydraulic cylinders, so that the rolls are pressed against the lower surface of the pressing plate. The desired pressure acts on the plastic sheet. In a second embodiment, strip-like heat conducting elements are inserted into the pressing plate. The heating elements are made from copper and contact the lower surface of the pressing plate. Several of these elements are provided in parallel. These in the whole are surrounded by a seal. Thus, the gap between the elements can be pressurized by a pressure medium. Oil and air are used as a pressure medium.
In this device, one disadvantage is that, due to the relatively large diameter of the deflecting rolls, the melt has to travel a relative long path until it reaches the area, where the press bands are supported by the pressing plates. Another disadvantage is, due to the strip-like copper strips or rolls, or the press-on elements, only a small contact area is provided to cool the metal belt and, in turn, the material sheet. Furthermore, the chosen support is not suitable to keep an accurate gap, as no accurate gap adjustment can be made with the pressure medium. Further in a construction where rolls, which only support line-like areas, are provided for the support, a gap adjustment can only take place in the areas between the rolls. This is especially disadvantageous with foamed plastics. Accordingly, sufficient surface quality cannot be achieved.
DE 196 05 951 A1 discloses a smoothing device. Here, the inlet gap facing the sheet die is formed between a smoothing roll and a metal belt. The belt runs continuously around two deflecting rolls. The two deflecting rolls are arranged next to the smoothing roll. A cooling unit can be arranged inside the continuous metal belt. This cooling unit can be provided in addition to deflecting rolls provided to serve as cooling rolls. Also, cooling units can be provided and the deflecting rolls without cooling possibilities.
U.S. Pat. No. 4,997,358 discloses a smoothing device. Several rolls are present to smooth and cool the melt liquid material sheet made from thermoplastic extruded from an extruder via a sheet die. The rolls are arranged in a frame. Since the plastics have to be processed into material sheets, an adjustment needs to be made in the guide for the desired tolerance of thickness. The holding time of the material sheet in contact with the rolls, a stationary roll and two adjustable rolls, can set by fixing the two adjustable rolls around the stationary roll at different angle positions by bolt connections. The fixing takes place via existing bores. The material sheet is guided after exiting the sheet die around part of the circumference of the rolls. Thus, during passing-over from one roll to the next, a change of mating surfaces takes place. First a cooling of one of the surfaces of the material sheet takes place. Next, a cooling of the other surface of the material sheet takes place. Furthermore, only angular contacts are possible which, in the maximum, amount to 180xc2x0 of the rolls.
One disadvantage of such embodiment is that an asymmetrical temperature profile can occur in the sheet. This leads to a worse transparency in plastic sheets or plastic film. Changes in the microstructure take place with the temperature change. Accordingly, different microstructures are achieved across the thickness of the film or sheet. This leads to the problem that no exactly flat sheets can be produced. The sheets show differences.
It is an object of the invention to provide a device for smoothing and cooling or cooling a material sheet extruded by an extruder via a sheet die. Accordingly, the device provides the possibility of producing sheets or film sheets with microstructures which stay the same over its thickness.
The device includes at least two units, one each acting upon one surface of the material sheet. The units provide smoothing and cooling, or cooling, and have respectively a base element. The base element has a flat guide surface and two curved deflecting faces continuous with both ends of the flat guide surface. The base also has a metal belt with an outer surface, a lower surface and a width. The width at least corresponds to the width of the material sheet. The metal belt is slidingly supported with its lower surface on the flat guide surface and slidingly guided around the deflecting faces. The outer surface of the metal belt, which is supported by the flat guide surface, forms a flat active area contacting the material sheet. The lower surface of the metal belt is at least partly coolable in a part below the flat active area contacting the surface of the material sheet. A driving roll is present over which the metal belt is guided and drivable. The drive roll drives the metal belt. A drive unit drives the drive roll.
An advantage of this embodiment is that the active area which contacts the material sheet is also designed to be even or flat so that while cooling or smoothing the material sheet, especially when plates are manufactured, no change in the microstructure takes place in a condition of formation, which does not correspond to the final condition, that is a straight plate. A double-sided arrangement relative to the material sheet or plate enables cooling and smoothing in opposite parts so that a symmetrical temperature profile can be achieved and the stress differences can be eliminated. Also, a high transparency is achieved in plastic films or plastic plates. If a larger cooling and smoothing path is required, several base units can be arranged one after the other along the direction of movement of the sheet to be treated. Once achieved conditions are maintained, due to the simultaneous double-sided cooling and smoothing of the material sheet, no temperature change takes place. Also, after exiting of the first station, which is formed of two units, until the entry into the following station, which includes two units, the appearing temperature rise in the peripheral zones takes place symmetrically. Accordingly, there is no difference in the microstructures towards the two surfaces of the sheet.
The small thickness of the metal belt makes tight deflection radii possible. Thus, small distances can be selected with an arrangement of several units one after the other. Accordingly, only small lengths of the material sheet to be treated between two subsequent units are not cooled. Thus, nearly a continuous double-sided cooling is possible over long lengths. Because of this it is possible that cooling lines, which commonly follow smoothing devices of the above described prior art, can be designed immensely shorter or even can be omitted.
An advantage of the solution according to the invention is that an area support is given. Thus, even with smoothing devices, on which foamed plastics are processed, flat (even) surfaces can be achieved. Accordingly, surfaces with fine cells can be achieved since the entry can be designed short due to the tight deflection radii of the metal belt on the base element. This means the uncooled free length, having exited the sheet die of the extruder, can be made very short. The deflecting faces allow a tight spatial allocation to the sheet die.
Preferably, the metal belt has a thickness of equal to or smaller than 0.6 mm. This enables an intensive cooling of the material sheet through a fast heat transfer from the sheet to the metal belt and to the means, which cool the lower surface of the metal belt. Especially advantageous conditions can be achieved when the belt thickness is equal to or smaller than 0.5 mm.
Advantageous cooling of the lower surface of the metal belt is achieved by the base element having cooling channels to transmit a cooling medium. The cooling channels are arranged in the guide surface and/or in the deflection faces of the base element transversely to the direction of the movement of the metal belt. The cooling channels open to the lower surface of the metal belt and are covered by the metal belt. Additionally, seals can be provided near the edges of the belt. The metal belt is slidingly supported on the seals. Alternatively, the cooling channels are in the form of bores in the base element. The bores are arranged in the base element close to the guide surface and/or to the deflecting faces and run transversely to the direction of movement of the metal belt.
An especially advantageous embodiment is achieved when at least two units are provided to form a pair. The two units are arranged opposite each other. The units are provided to take up a material sheet between them. To achieve an adjustment to different material sheet thicknesses, at least one of the units is adjustably mounted in the direction to the other unit. When required, several more pairs of units can be arranged one after the other, along the direction of movement of the material sheet. To achieve optimum conditions, it is further possible that at least one of the units or at least one pair of the units is adjustably arranged along the direction of movement.
From the following detailed description, taken in conjunction with the drawings and subjoined claims, other objects and advantages of the present invention will become apparent to those skilled in the art.